I plan to make a guacamole dip and one of the ingredients is Serrano pepper, which is 10,000 to 23,000 heat units by the Scoville scale.
That is way too spicy for me.
Which one of the milder peppers (100-2,000) would be a good replacement?
US Jalapeno and Anahiems (red jalapenos) are usually under 2,500. If you cook them a little bit, it will also reduce the spiciness, while maintaining the flavor.
Anaheim peppers make a nice substitute, first in reducing the heat (500-5000 scoville), but also in terms of its availability and similar flavor. It does frequently run on the hotter end of that spectrum though.
You can just omit them if you don't want any heat. Otherwise, Anaheim or Jalapeno would be fine choices.
Chopped green peppers are a great substitute if you absolutely don't want any hotness to your dish. I used poblano peppers are a great substitute and they register as a'1' on the hotness scale.